perhaps i am too late to answer your question and if so just ignore the following:

here are three screen shoots that may be of help to guide you by selecting an instrument track first then selecting a loop to apply to that track from the " loops " drop down menu. I used the piano track for this explanation.

There is also more info/details in the built in manual ( go to the drop down Help in the Band in a Box menu bar ).

Hope that i have not made it more complicated then it really is

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Mac Audiophile 2020 2019 2018 2017 USB plus previous years versions

macOS Mojave - 10.14.6 @ Aug 16, 2019

iMac i3 3.6 GHz 8GB RAM 4K with Retina display

some examples @ sound cloud done with Mac Band and DAW's : https://soundcloud.com/you/tracks